Signaling system for drilling

ABSTRACT

A pressure pulse generator for use in transmitting pressure signals to surface in a fluid-based drilling system. The generator is arranged in use in the path of a pressurized fluid to operate a drilling assembly and is capable of being actuated to generate pressure signals in such fluid for transmission to surface pressure monitoring equipment. The pulse generator includes pulse height compensation to keep the pulse height within acceptable limits over a wide flow range.

[0001] This invention relates to a system of communication employedduring the drilling of boreholes in the earth for purposes such as oilor gas exploration and production, the preparation of subterraneanservices ducts, and in other civil engineering applications.

BACKGROUND TO INVENTION

[0002] Taking the drilling of oil and gas wells as an example, it ishighly desirable both for economic and for engineering reasons, toobtain information about the progress of the borehole and the stratawhich the drilling bit is penetrating from instruments positioned nearthe drilling bit, and to transmit such information back to the surfaceof the earth without interruption to the drilling of the borehole. Thegeneric name associated with such techniques is“Measurement-while-Drilling” (MWD). Substantial developments have takenplace in MWD technology during the last twenty-five years.

[0003] One of the principal problems in MWD technology is that ofreliably telemetering data from the bottom of a borehole, which may lieseveral thousand meters below the earth's surface. There are severalestablished methods for overcoming this problem, one of which is totransmit the data, suitably encoded, as a series of pressure pulses inthe drilling fluid; this method is known as “mud pulse telemetry”.

DESCRIPTION OF PRIOR ART

[0004] A typical arrangement of a known mud pulse MWD system is shownschematically in FIG. 1. A drilling rig (50) supports a drillstring (51)in the borehole (52). Drilling fluid, which has several importantfunctions in the drilling operation, is drawn from a tank (53) andpumped, by pump (54) down the centre of the drillstring (55) returningby way of the annular space (56) between the drillstring and theborehole (52). The MWD equipment (58) that is installed near the drillbit (59) includes a means for generating pressure pulses in the drillingfluid. The pressure pulses travel up the centre of the drillstring andare received at the earth's surface by a pressure transducer (57).Processing equipment (60) decodes the pulses and recovers the data thatwas transmitted from downhole.

[0005] In one means of generating pressure pulses at a downholelocation, the fluid flowpath through the drillstring is transientlyrestricted by the operation of a valve. This creates a pulse, theleading edge of which is a rise in pressure; hence the method iscolloquially, although rather loosely, known as “positive mud pulsetelemetry”. In contradistinction the term “negative mud pulse telemetry”is used to describe those systems in which a valve transiently opens apassage to the lower pressure environment outside the drill string, thusgenerating a pulse having a falling leading edge.

[0006] Devices for generating pulses for positive mud pulse telemetryhave been described in, for example, U.S. Pat. Nos. 3,958,217,4,905,778, 4,914,637 and 5,040,155.

[0007] The present invention is related generally to the type of mudpulse generator described in U.S. Pat. No. 3,958,217. It is adisadvantage of this type of pulse generator that the magnitude of thetransient pressure change which occurs downhole is highly dependent onthe flowrate of the drilling fluid.

[0008] The pressure drop when fluid flows through a restriction variesapproximately as the square of the flow rate. Typically, the ratio ofmaximum to minimum flow rates in an oilwell drilling situation is aroundthree, so a pulse generator set up to give an acceptable pulse height ofaround 7 bar at minimum flow of a particular drilling mud formulationwould give 63 bar at maximum flow. In practice, drilling mud isformulated with a wide range of densities and viscosities, so thepotential variation in pulse height across the flow range isconsiderably greater.

[0009] Although in any given drilling situation a certain minimum pulseamplitude is needed so that the pulse will be detectable at the earth'ssurface, it is unsatisfactory for the pulse to be made too large: theimposition of a succession of severe flow restrictions can stress,damage or erode the drilling equipment and starve the drilling bit offluid. Furthermore, when mud pressure pulses are too large, significantpulse reflections occur at discontinuities in the process pipework. Inparticular a pulse can return to the lower end of the drillstring, bereflected, return to surface and be detected, incorrectly, as a datapulse.

[0010] In order to keep pulse heights within acceptable limits, thepulse generator has to be physically adjusted to suit a particularcombination of flow rate and mud type. This typically involves replacingparts of the downhole system, and is time consuming and expensive. Thereare cases too, in which for unexpected reasons, the planned flowratesfor a particular well section have to be changed while the equipment isdownhole. It is therefore very desirable to provide a single systemwhich will operate satisfactorily over a wide range of drilling fluidflowrates.

[0011] The invention seeks to obtain this advantage by providing a meansof automatic pulse height regulation in the fluid used in a drillinginstallation.

SUMMARY OF INVENTION

[0012] According to the invention there is provided a pressure pulsegenerator for use in transmitting pressure signals to surface in afluid-based drilling system, said generator being arranged in use in thepath of a pressurised fluid to operate a drilling assembly and beingcapable of being actuated to generate pressure signals in such fluid fortransmission to surface pressure monitoring equipment, in which thepulse generator comprises:

[0013] a housing positionable in the path of the supply of pressurisedfluid, said housing having an inlet arrangement for admitting a portionof the fluid to the interior of the housing, and an outlet arrangementfor discharging fluid from the interior of the housing;

[0014] a control element slidably mounted in the housing for movementbetween an open position and a closed position with respect to saidinlet arrangement, said control element being operative to generate apressure pulse in the supply of pressure fluid when the control elementtakes-up the closed position;

[0015] a control passage for receiving a portion of the supply ofpressure fluid and extending through the control element, and having aninlet at one end to receive pressure fluid and a discharge outlet at anopposite end;

[0016] a valve element arranged to be exposed to the pressure of thefluid in the control passage;

[0017] an actuator coupled with the valve element and operative to movethe valve element between a closed position in which it preventsdischarge of pressure fluid from the control passage, and an openposition in which it allows the pressure fluid to flow through thecontrol passage;

[0018] a control face on the control element which is exposed to thepressure of the fluid in the control passage and which is operative tomove the control element towards the closed position with respect to theinlet arrangement as the pressure in the control passage increases uponmovement of the valve element to the closed position by the actuator;and,

[0019] a resiliently yieldable arrangement acting between the actuatorand the valve element in order to define a yieldable limit to thepressure of the fluid in the control passage and thereby control thepressure pulse generated by the movement of the control element to theclosed position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 2 is a longitudinal sectional view of a pressure pulsegenerator according to the invention, located downhole and in the pathof a pressurised flow of fluid (mud) to operate a drill located belowthe pulse generator, and showing the generator in an inoperative mode,allowing throughflow passage of the fluid, without generating anypressure pulse signals to surface;

[0021]FIG. 3 is a view, similar to FIG. 2, but showing the movement ofthe internal components of the generator to a pressure signaltransmitting mode, after actuation of the generator to block throughflowof fluid; and,

[0022]FIG. 4 is a view, similar to FIGS. 2 and 3, but showing theinternal components in a partly closed position, whereby to reduce, whennecessary, the magnitude of the pressure of the signaling pulsegenerated.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0023]FIG. 2 shows a mud pulse generator, designated generally byreference 100, and mounted in a drill collar (1). The pulse generator isgenerally of the type described in U.S. Pat. No. 3,958,217, in which theenergy needed to operate the restricting valve is derived from thedrilling fluid. Drilling fluid flows down through the space and passagesin the bore of drill collar (1), on through a drilling motor (if fitted)and thence to the drill bit (not shown). The drilling fluid returnsupwards in the annular space between the outside of the drill collar (1)and the rock formation being penetrated (not shown). In a typicalinstallation the fluid is “drilling mud”. However, other fluids may beused, including gas, foam or mist.

[0024] A housing is positioned in the path of the pressurised drillingfluid and comprises a body (10), located inside the drill collar (1) andhaving three different internal bores (6), (7) and (9).

[0025] A control element in the form of piston (26) is a sliding fit inthese bores. Its upward travel is limited by the face (25) at the upperend of the largest bore (9). Its downward travel is limited by the face(27) of the mounting (11).

[0026] Inlet and outlet arrangements comprise inlet orifices (21) andexit orifices (8) provided in the body (10). Mud can flow along the path(5) through these orifices except when the piston (26) is in the fullyforward (upward) position.

[0027] A screen (2) perforated by holes or slots (19) is retained at thefront of the body (10) by a nose cone (18). Drilling fluid can normallyflow also along a control passage comprising second path (20) throughthe screen holes (19), ports (3) in the body (10), and a central bore(4) in the piston (26). The dimensions of the holes or slots (19) arechosen to prevent blockage of the central bore (4) by mud particles.

[0028] A valve element (13) connected to an actuator (17) is normallyheld clear of its seat (28) in the mounting (11) to permit flow alongthe path (20) past the valve element (13) and out through ports (12) inthe mounting (11).

[0029] A fixed restrictor (22) supporting the front of the body (10)contains ports (23) to provide a third flow path (24) outside the body.The mounting (11) has ports (16) to permit flow to continue down thedrill collar.

[0030] The basic operation of the pulse generator will now be described.

[0031]FIG. 2 shows the pulse generator in the normal, off pulsecondition. Drilling fluid flows along the three paths (5), (20) and(24). The pressure upstream of the restrictor (22) is higher than thatdownstream because of the throttling effect of the restrictor (22) onthe mud flow. The piston (26) is held in the rearward (bottom) positionby flow forces and by the differential pressure created by therestrictor (22).

[0032] To initiate a pulse, the valve (13) is closed by the actuator(17). High pressure flow from the region upstream of the restrictor (22)transmitted along path (20) now builds up between the piston (26) andthe face (27) of the mounting (11). The area of face (27) is greaterthan the area of the piston in bore (6) which is directly exposed to theupstream pressure. The net force on the piston (26) is now in theupwards direction and the piston moves upwards until its travel isstopped by contact with face (25).

[0033]FIG. 3 shows the piston (26) in the fully forward position withthe valve (13) still closed. Flow is now only along path (24), and thepressure drop across the pulse generator is entirely determined by thearea of the restrictor ports (23), the mud flow rate, density andviscosity. This pressure drop will be maintained for as long as thevalve (13) is held on the seat (28).

[0034] To return to the initial conditions as shown in FIG. 2, the valve(13) is withdrawn from the seat (28) by the actuator (17) e.g. byde-energising of the actuator (17). Pressure behind the piston (26) isreleased, so that the net force on the piston is once again in thedownwards direction. The piston (26) moves back to its original positionunder the influence of this downwards force, assisted by flow forcesonce the exit orifices (8) start to re-open.

[0035] It can be seen that with the valve (13) fully in contact with theseat (28), the only way of altering the on-pulse pressure drop would beto change the area of the ports (23) in the restrictor (22).

[0036] A particularly advantageous further feature of the pulsegenerator will now be described, and its mode of operation.

[0037] A resilient biasing arrangement acts between the valve (13) andthe actuator (17), and in the illustrated embodiment takes the form of aspring 15 (or other compliant element). The spring (15) is contained ina housing (31) and acts against an increased diameter section (30) ofthe rod (14) connected to the valve (13). Movement of the rod (14) islimited by a reduced diameter (29) at the upstream end of the housing(31). The housing is attached to the output rod (33) of the actuator(17) by a coupling (32) which also provides the rear abutment for thespring (15).

[0038] When the actuator (17) is operated to initiate a pulse, the valve(13) is forced against the seat (28) through the intermediary of thespring (15). The piston (26) moves forward as previously described, andas it does so, the flow along path (5) is increasingly throttled as theexit orifices (8) are blanked off by the piston.

[0039] The resultant increased pressure drop across the pulse generatoris transmitted along path (20) to the valve (13). If the pressure dropbecomes sufficiently high to overcome the spring force, the valve (13)is forced off the seat (28) and a certain amount of flow isre-established along path (5). A situation is reached as shown in FIG. 1where the forces on the piston (26) and the valve rod (14) are inequilibrium. The piston (26) and the valve stem (13) are in intermediatepositions, and the pressure drop across the mud pulse generator istherefore determined by the characteristics of the spring (15).

[0040] With a suitable choice of stiffness and initial compression, thepulse height can be kept within acceptable limits over a wide flowrange.

[0041] The restrictor (22) may be changed to keep the flow rate alongpath (5) within the control range of the spring (15) if a major changein total flow rate is to occur.

[0042] In a preferred embodiment, the parts of the pulse generator aremade from materials suitable for the environment of deep drillingoperations. As is well-known to those who work in this field, materialssuch as beryllium-copper and stainless steel are suitable materials forparts of the system which contact the drilling fluid. In regions of thesystem where fluid velocities are high, it is preferable to employespecially hard material, such as tungsten carbide, for good resistanceto fluid erosion. The actuator (17) is a conventional electromagneticsolenoid. It is well-known, and good practice, to isolate items such asthe actuator (17), the spring (15) and the associated parts, from directcontact with the drilling fluid. This is typically done by employingresilient seals to provide isolation and then filling the space soenclosed with a light hydraulic oil. These details have been omittedfrom the drawings for clarity.

[0043] Using a mud pulse generator with pulse height compensation builtaccording to this invention, tests were carried out using a flow loop todetermine the efficacy of the pulse height compensation. The followingresults were obtained in a representative test. Height of pressure pulsein the Drilling fluid flow absence of Height of pressure rate (USgallons compensation pulse with per minute) (bar) compensation (bar) 200 6.6 6.9 300 14.4 7.3 400 28.3 7.6 500 * 6.9 600 * 7.7

I claim:
 1. A pressure pulse generator for use in transmitting pressuresignals to surface in a fluid-based drilling system, said generatorbeing arranged in use in the path of a pressurized fluid to operate adrilling assembly and being capable of being actuated to generatepressure signals in such fluid for transmission to surface pressuremonitoring equipment, in which the pulse generator comprises: a housingpositionable in the path of the supply of pressurized fluid, saidhousing having an inlet arrangement for admitting a portion of the fluidto the interior of the housing, and an outlet arrangement fordischarging fluid from the interior of the housing; a control elementslidably mounted in the housing for movement between an open positionand a closed position with respect to said inlet arrangement, saidcontrol element being operative to generate a pressure pulse in thesupply of pressure fluid when the control element takes-up the closedposition; a control passage for receiving a portion of the supply ofpressure fluid and extending through the control element, and having aninlet at one end to receive pressure fluid and a discharge outlet at anopposite end; a valve element arranged to be exposed to the pressure ofthe fluid in the control passage; an actuator coupled with the valveelement and operative to move the valve element between a closedposition in which it prevents discharge of pressure fluid from thecontrol passage, and an open position in which it allows the pressurefluid to flow through the control passage; a control face on the controlelement which is exposed to the pressure of the fluid in the controlpassage and which is operative to move the control element towards theclosed position with respect to the inlet arrangement as the pressure inthe control passage increases upon movement of the valve element to theclosed position by the actuator; and, a resiliently yieldablearrangement acting between the actuator and the valve element in orderto define a yieldable limit to the pressure of the fluid in the controlpassage and thereby control the pressure pulse generated by the movementof the control element to the closed position.
 2. A pressure pulsegenerator according to claim 1, in which the resiliently yieldablearrangement comprises a compression spring.
 3. A pressure pulsegenerator according to claim 2, in which the actuator is coupled withthe valve element via a housing in which the compression spring isarranged, and which spring acts between the actuator and an actuator rodslidably mounted in the housing and coupled at one end with said valveelement.
 4. A pressure pulse generator according to claim 1, in whichthe actuator is electromagnetically operable.
 5. A pressure pulsegenerator according to claim 2, in which the actuator iselectromagnetically operable.
 6. A pressure pulse generator according toclaim 3, in which the actuator is electromagnetically operable.
 7. Apressure pulse generator according to claim 1, in which the housing hasexternal ports, which allow by-pass flow of the supply of pressurizedfluid.
 8. A pressure pulse generator according to claim 2, in which thehousing has external ports, which allow by-pass flow of the supply ofpressurized fluid.
 9. A pressure pulse generator according to claim 3,in which the housing has external ports, which allow by-pass flow of thesupply of pressurized fluid.
 10. A pressure pulse generator according toclaim 4, in which the housing has external ports, which allow by-passflow of the supply of pressurized fluid.
 11. A pressure pulse generatoraccording to claim 5, in which the housing has external ports, whichallow by-pass flow of the supply of pressurized fluid.
 12. A pressurepulse generator according to claim 6, in which the housing has externalports, which allow by-pass flow of the supply of pressurized fluid. 13.A pressure pulse generator according to claim 7, in which at least oneof the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.14. A pressure pulse generator according to claim 8, in which at leastone of the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.15. A pressure pulse generator according to claim 9, in which at leastone of the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.16. A pressure pulse generator according to claim 10, in which at leastone of the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.17. A pressure pulse generator according to claim 11, in which at leastone of the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.18. A pressure pulse generator according to claim 12, in which at leastone of the external ports is replaceably mounted, to allow a replacementport to be installed having a different pressure restriction and therebyto adjust the pressure of fluid passing to the interior of the housingwhen a major change in flowrate of the pressurized fluid is to occur.